We propose to study cortical organization and reorganization at the level of neuronal networks and assemblies, using both experimental and theoretical methods. The experimental vehicle will be rat auditory cortex subjected to local electrical stimulation; in analogy to work done by others as well as in our laboratory on somatosensory cortex, this procedure should force changes in the auditory frequency and ear dominance 'map' arrangement. During such changes we propose to make extracellular recordings simultaneously and separably from 10-30 neurons in the reorganizing cortical region. These measurements and their subsequent interpretation in terms of neuronal assembly processes will rest on recording technologies and analytic mathematics that have largely been developed by our laboratory over the last decade. The proposed theoretical work will examine computer simulations of neuronal networks arranged to reproduce the changes in map magnification and boundaries which underlie the experimental work. As information about 'effective connectivity' and neuronal assembly properties emerges from the experiments. these now constraints will be incorporated into the ongoing modeling. The models will be studied on both a gross level, and on a 'microscopic' scale, using our standard spike train analysis techniques, and are likely to suggest additional experimental measurements. The overall project should give considerable insight into actual and possible mechanisms for cortical modularity and reorganization; we will here have direct experimental access to the details of the process by which the brain allocates its computational resources. Comparison of the auditory work to the analogous somatosensory project will test the extent to which the reorganization processes are universal across sensory systems, or conversely whether they will prove to be unique.